Function useViewerSnapshot

The plumbing every state hook in this package shares: subscribe to viewer events, re-read a value when they fire, and feed the result to React through useSyncExternalStore so it behaves under concurrent rendering.

Two details make this work with object-shaped snapshots, which useSyncExternalStore otherwise loops on:

• the value is recomputed only after a notification, not on every render;
• isEqual lets a recomputed-but-unchanged value keep its previous identity, so React can bail out of the re-render.

The viewer can be null (before PdfViewerSurface mounts, and during SSR), so getSnapshot must handle that.